1. Field of the invention
The present invention relates to a light projector for a distance measuring device and more particularly to a light projector having an adjustment mechanism for adjusting the inclination of a flattened light beam which may be continuous or segmental.
2. Prior Art
There are known automatic focusing cameras which have a subject distance measuring device and a lens positioning device for positioning a taking lens according to output signals from the subject distance measuring device. A compact camera generally uses a subject distance measuring device of a so-called active type which measures the subject distance by projecting a light beam onto the subject, receiving the light reflected from the subject, and detecting the position of the light incident on a light sensor. There are two types of such an active type subject distance measuring device (which is hereinafter referred to as an "active type device"). One type projects a light beam having a circular cross section; whilst another projects a light beam (which is hereinafter referred to as a "slit light beam") having a rectangular cross section formed by a slit whose longitudinal direction is perpendicular to the base line of the active type device. The latter has an advantage that it can accurately measure the subject distance without performing a prefocussing operation even if the subject to be photographed is not in the center of the area in the viewfinder, such as when the subject comprises two or three persons standing side by side.
FIG. 6 shows a conventional automatic focusing camera having an active type device which projects such a slit light beam. A camera body 1 has a taking lens 2 in its front which is precisely set at a position corresponding to a measured subject distance by the lens positioning device. The camera body 1 also has a light projector 4 and a light receiver 5 with the taking lens 2 therebetween. The line passing through the centers of the light projector 4 and light receiver 5 is the base line BL for measuring the subject distance, and the distance between the centers of the light projector 4 and light receiver 5 is the base length.
FIG. 7 is a schematic illustration of a subject distance measuring device. The light projector 4 comprises a rod-like light source 8, a slit plate 9, and a projecting lens 10. Toward a subject 11 or a subject 12, the projector 4 projects a slit light beam of which the longitudinal direction of the cross section is perpendicular to the base line BL. The light receiver 5 comprises a light receiving lens 16 and a light sensor array 17 and photoelectrically converts received light which is part of the received slit light beam reflected from the subject 11 or 12. The light sensor array 17 comprises a plurality of rectangular photosensors 17a which are placed in a row parallel to the base line BL. Since the light reflected from the subject 11 or 12 is received by that one of photosensors that corresponds to the subject distance, the subject distance can be measured approximately by detecting which photosensor generates a photoelectrically converted signal. The lens positioning device positions the taking lens 2 according to the measured subject distance.
In a distance measuring device using a slit light beam, it is necessary that the longitudinal direction of the slit light beam be perpendicular to the base line BL as described above, and the longitudinal direction of each photo sensor 17a must also be perpendicular to the base line BL. If the longitudinal direction is not perpendicular to the base line, precise distance measurement is not possible because the reflected light is not received by the photosensor corresponding to the subject distance.
Conventionally, the inclination of the slit plate with its slit is adjusted so that the longitudinal direction of the slit is perpendicular to the base line BL, by means of a spring which biases the slit plate and two screws which contact the slit plate biased by the spring. However, this adjusting method has a serious problem in that it is quite difficult to adjust the inclination of the slit plate by rotating the screws alternately, keeping the center of the slit aligned with an optical axis 13a of the light projector 4. It consequently takes too long a time to adjust the device. Moreover, because the slit plate is supported at three points, there is another problem in that the number of parts increases and the projector becomes larger in size, thus raising the manufacturing cost.